As is known in the art, one type of conventional current sensor uses a magnetic field transducer (for example a Hall effect or magnetoresistive transducer) in proximity to a current conductor. The magnetic field transducer generates an output signal having a magnitude proportional to the magnetic field induced by a current that flows through the current conductor. Typically, the current sensor also includes circuitry to amplify and condition the output signal of the magnetic field transducer.
The magnetic field transducer and circuitry are sometimes provided as an integrated circuit (IC) in an IC package that also contains a current conductor. Illustrative current sensors of this type are sold under part numbers ACS712 and ACS758xCB by Allegro MicroSystems, Inc. of Worcester, Mass. 01615, the Assignee of the subject application.
Various parameters characterize the performance of current sensors, including sensitivity. Sensitivity is related to the magnitude of a change in output voltage from the Hall effect transducer in response to a sensed current. The sensitivity of a current sensor is related to a variety of factors. One important factor is the physical separation between the Hall effect element and the current conductor.
Integration of the current conductor into an IC package allows for close and precise positioning of the current conductor relative to the magnetic field transducer. However, the amount of current that can be routed through the current conductor is limited by the physical and thermal limitations of the IC package.
One technique for measuring current levels that exceed the current carrying capability of the current sensor is to physically split the current path between the integrated current conductor and an external shunt current conductor that is coupled in parallel with the integrated current conductor. This technique can also be used to increase the current transient survival capability of the current sensor (also referred to as the current sensor's “over-current capability”) by diverting some current away from the current sensor. With such a current divider arrangement, only a portion of the total current to be measured is carried by the integrated current conductor and the remainder of the current is carried by the external shunt current conductor. The external shunt current conductor can be implemented with a trace or layer on a printed circuit board (PCB) on which the current sensor is mounted or with a bus bar. Current conductors that are configured as current dividers are designed to achieve a known division of current so that measurement of the current carried by the integrated current conductor can be used to determine the total current.
While this type of arrangement can be used to increase current measurement levels of an application as well as current carrying capability of the current sensor, there are disadvantages. Namely, since less current will flow through the integrated current conductor, the resulting magnetic field signal level (i.e., the resolution) will be lower. Also, manufacturing and assembly tolerances, both at the device and at the board level, result in some variability in the division of current between the integrated current conductor and the shunt current conductor from the designed division of current. For example, the resistance of the current sensor package's lead frame can vary over the course of time due to production tolerances. The manufacturing process of soldering the current sensor package leads to a PCB trace is also very important, as higher solder resistance can cause more current to pass through the shunt conductor and less through the integrated current conductor of the current sensor. The thickness and width of PCB traces can also vary as a function of manufacturing tolerances. Such variability is undesirable since the total current cannot be accurately determined from the measurement of current carried by the integrated current conductor. In applications where accuracy requirements make it necessary to compensate for these variations, some current sensors allow the sensitivity to be programmed after assembly as is described in an Application Note AN295036, Rev. 3, of Allegro MicroSystems, Inc. entitled “Using Allegro Current Sensor ICs in Current Divider Configurations for Extended Measurement Range” (and published at http://www.allegromicro.com/en/Products/Design/an/an295036.pdf), which Application Note is incorporated by reference herein in its entirety.